When, where and how does Visual Studio 2017 set the DOCKER_BUILD_SOURCE environment variable

Question

When creating a new .NET core application with docker support in Visual Studio 2017 it creates a number of docker-compose.yml files. The docker-compose.vs.debug.yml and the release variant both contain contain a reference to an environment variable named DOCKER_BUILD_SOURCE:

version: '2'  services:   app:     image: app:dev     build:       args:         source: ${DOCKER_BUILD_SOURCE}     environment:       - DOTNET_USE_POLLING_FILE_WATCHER=1     volumes:       - ./app:/app       - ~/.nuget/packages:/root/.nuget/packages:ro       - ~/clrdbg:/clrdbg:ro     entrypoint: tail -f /dev/null     labels:       - "com.microsoft.visualstudio.targetoperatingsystem=linux"  The purpose of this variable seems to be a reference to the source directory, however, it always seems to be empty. 

I was unable to find more detailed info on this subject... Does anyone have an idea or a pointer to some docs?

Answer 1

I believe it has to do with doing some setup that can be used with Visual Studio Team Services CI/CD. However when run locally, that value is empty and if you look at the docker file, you see that if the value is empty, it substitutes "obj/Docker/publish"

Dockerfile:

FROM microsoft/aspnetcore:1.0

ARG source

WORKDIR /app

EXPOSE 80

COPY ${source:-obj/Docker/publish} .

ENTRYPOINT ["dotnet", "app.dll"]

However for me, i don't actually see that folder or anything in it. Where the "magic" happens is in the volumes section. that essentially moves your code onto the container as a bind mound. this is where your code gets moved onto the container. There are a few other things that happen that aren't clear to me though because I see a line in the build output where the code gets built/published, but not the actual command that is running.